import inspect (#3507)

Inspect is used five times on line 65-74 without being imported.
diff --git a/python/stubout.py b/python/stubout.py
index aee4f2d..ba39104 100755
--- a/python/stubout.py
+++ b/python/stubout.py
@@ -17,6 +17,9 @@
 # This file is used for testing.  The original is at:
 #   http://code.google.com/p/pymox/
 
+import inspect
+
+
 class StubOutForTesting:
   """Sample Usage:
      You want os.path.exists() to always return true during testing.